There are conventionally known premixed-combustion gas turbine combustors that previously mix the fuel with combustion air and then burn the premixed gas (for example, see Patent Literature 1). The gas turbine combustor includes a main burner in which the premixed gas passes. The main burner includes a burner external cylinder and an extension pipe placed downstream of the burner external cylinder. Burning the premixed gas from the main burner generates flashback that is a phenomenon of a backfire (combustion) in the main burner. In light of the foregoing, to prevent the generation of flashback, an air formed into a film shape (the film air) flows from the gap between the burner external cylinder and the extension pipe and along the inner wall surface of the extension pipe.